The first and one of the most important modifications for an XR is to use a K&N air filter. This filter will flow the same as a brand new paper element, but the real improvement is the K&N will flow for 1000's of miles while paper filters quickly get clogged. Don't forget, our motors breath like a good running 5.0 liter (we make 260 hp, right).
I did a bit of testing with the 6" K&N cone filter yesterday, and here's what I found...
I did this with my '87 XR4Ti, which is currently 100% stock, and my G-Tech in the HP mode. (no roads close by to do some 1/4 mile passes).
Test 1- Stock airbox, with the factory replacement K&N filter I've had in my SVO for years. In this configuration cool air is drawn from inside the fender. I did 5 runs and averaged 140HP, so if you add 20% for driveline loss that works out to 170HP. Seems about right for the 175HP Ford advertised new.
Test 2 - Stock airbox and filter, except the top cover has had the sides removed, It seals the top of the element, but the rest is open to the engine compartment. In this configuration cool air is drawn from the fender, as well as hot air from the engine compartment. Here I averaged 137 HP.
Test 3- I put the 6" K&N cone directly on the front of the air meter. Now air is being drawn from the hot engine compartment only. Here I averaged only 131.5 HP.
Test 4 - Setup returned to stock, resulted 140 HP. This means that the testing is consistant.
Analysis -It seems that with the stock setup COOL air is BETTER than INCREASED FLOW. Although keep in mind that this was done with the small VAF meter. Which probably flows about the same as the stock airbox. I'm going to try the large meter from my SVO, which is currently in pieces getting Volvo bits grafted to its intake. I suspect the larger meter will then show the stock airbox as being the restriction. Anyhow I've convinced myself that fabing up some ducting to get air from outside the engine compartment is the way to go, along with the cone filter. I'll share my results of the large VAF tests.
When prepairing for the SVO VAF/EEC I decided to increase the air flow in the air box part of the induction system. After looking at Jon Welsh's dyno data (in the Encylopedia), I feel that the air box has lots of room for improvement. My step by step mods are below.
AIR BOX INLET
I got an extra air box and cut out the snout part off a lid. I then cut a hole in a base (on the fender side) and welded the snout in. I got a plastic snout and shortened it. I also cut an oval hole in the fender right below and slightly forward of the existing hole. Now I have TWICE AS MUCH COLD AIR CAPICITY into the air box.
AIR BOX OUTLET
I got a 3" tube from a muffler shop and had them chamfer the end about 1/2" (on the hydrolic tube bender). I cut the piece about 3" long. I took the air box base and cut out the 2.6" dia tube and welded in the 3" dia part. This enabled me to use a 3" piece of aluminum house air duct to connect the VAF to the air box base. The rubber hose fits nice but looks to have pretty bad flow. The 3" duct has smooth curves, it has 19% increase in area.
I won't be able to judge the imrovement since I now have the SVO VAF/EEC at the same time as the air box mods. Seems to run good on a very limited test.